In insulating liquid filled power bushings for high voltage apparatus such as oil filled bushings for transformers, the oil in the oil filled bushings is in contact with a cushion of gas that allows the oil to expand and contract due to thermal changes. When the oil expands, the gas is compressed and when the oil contracts, the gas expands to fill the space. As the pressure in the gas space varies and the temperature varies, the tendency for the oil to absorb and contain gas changes. As a result, gas is cyclically entering and leaving the oil over the life of the bushing. In some cases, the giving up of gas is so quick that it results in bubbles forming in the oil below the surface. These bubbles can occur in locations of high stress and produce partial discharge, which interferes with transformer testing and may reduce the life of the bushing in severe cases.
It would be desirable to provide an oil filled bushing where the oil is prevented from contacting the gas cushion. In this way, the oil can't absorb gas and remains devoid of large quantity of gas which make gas bubble evolution impossible.